The effects of aging on the capacity of the gastrointestinal (GI) tract to absorb drugs are not known. A number of anatomical and physiological changes in the GI tract are known to occur with aging: gastric pH increases, intestinal blood flow decreases, intestinal epithelial cell population declines, and GI motility may be affected. Such changes are known to affect drug bioavailability and several authorities have speculated that drug bioavailability may be different in elderly patients than in younger patients. To assess the effects of aging on GI drug absorption, the oral bioavailability of four test drugs will be determined in senescent (30 month-old) and control (12 month-old) rats. The four test drugs (tetracycline, sulfisoxazole acetyl, bretylium, and riboflavin) tend to be poorly absorbed under the best of circumstances; since each drug has a unique bioavailability problem (slow dissolution, low mucosal permeability, saturable transport mechanism), a different aspect of GI absorptive capacity is tested by each drug. The following measures of bioavailability will be made: extent of absorption, rate of absorption, and intestinal mucosal permeability. Experiments are also planned to measure gastric motility, intestinal blood flow rate, and gastric pH in senescent and control rats so that mechanisms for altered drug bioavailability may be established.